The present invention relates to an apparatus for collecting fine particles from fluid streams and, more particularly, to a thermal precipitator having closely spaced apart, opposing hot and cold surfaces which cause such particles in a fluid stream in the spacing between the surfaces to undergo thermophoretic movement and precipitate on the cold surface of the thermal precipitator. The invention also relates to a method of using the thermal precipitator to collect or remove fine particles from fluid streams.
Sampling fine particles, i.e. those having less than 10 micron aerodynamic equivalent diameter (AED), from a fluid stream such as ambient air can be accomplished through a variety of methods. Dry cyclones, wet cyclones, scrubbers, impactors, and filters are a few methods conventionally available, but may have shortcomings in certain applications because they can affect the physical characteristics of the particles during the collection process. These physical alterations include mixing, spalling, agglomerating, compressing, dissolving or embedding of the particles into the collection medium. While such particle modifications may be acceptable if the particles are to be analyzed in bulk, they may not allow observers to categorize and examine the particulate matter as it is present in the atmosphere or other fluid stream. In addition, when collecting live biological particulate material, the analysis may require that the material remain viable. Many of the particle collectors referenced above apply stress to the biological material such that some or all of it is no longer biologically active. Moreover, these collectors have difficulty collecting ultrafine particles, i.e. less than 1 micron AED, making them poorly suited for applications where collection of ultrafine particles is required. Other disadvantages experienced by at least some of these collectors include pressure build-up as particles deposit on the collection surface, loss of collection fluid over an extended sampling time, and particle charging effects.
Conventional thermal precipitators provide a promising approach for collecting fine particles from air streams. Thermal precipitators have been documented to collect over 99.9 percent of particles less than 5 microns in diameter. Unlike other types of collectors, a thermal precipitator works better as the particle size decreases. Efficient collection of particles as low as 0.01 microns has been shown. Thermal precipitation is generally efficient for collecting articles smaller than 10 microns in diameter.
A thermal precipitator typically consists of a cooled plate and a heated plate separated by a very small distance that permits air containing suspended particles to flow in the space between the plates. At least a portion of the suspended particles precipitates on the surface of the cooled plate as a result of thermophoresis, a phenomenon where the kinetic energy of the air molecules drives the suspended particles from hotter areas to colder areas. Because of the temperature gradient between the cooled and heated plates, the net transfer of energy from the air molecules to the suspended particles tends to propel the particles from the warm (high energy) level to the cooler level, resulting in precipitation of the particles on the cooled plate. The migration of the suspended particles resulting from the thermal motion of the fluid molecules is referred to as Brownian movement.
The thermal precipitator collection method is very gentle to the collected particles in comparison to other collection methods and is not limited by buildup of high pressure as particles are collected, by particle impact and loss of viability of the collected particles, by loss of collection fluid over an extended length sampling time, or by particle charging effects. Moreover, particles are segregated by size on the cool particle collection surface, as small particles are precipitated first and larger particles are precipitated later. Only moderate temperature differences are needed to construct an efficient thermal precipitator.
One notable disadvantage of known thermal precipitator designs is the tendency to be bulky and cumbersome to transport and use. Thermal precipitators typically use water to provide cooling of the cooled plate and electrical resistance heating to provide heating of the heated plate. While this construction may be acceptable for use at fixed locations where water and electrical service are present, it severely limits the suitability of conventional thermal precipitators for use in remote, mobile or personal monitoring applications where small size, ready transportability and/or self-contained heating and cooling capability are required.
Thermoelectric modules are used to provide alternately cooled and heated surfaces using the physical principle, called the xe2x80x9cPeltier Effect,xe2x80x9d where a direct current applied to a junction of two dissimilar materials causes one junction of the circuit to become cold while the other junction becomes hot. Practical considerations require that the two junction materials be metallic semiconductors. A variety of solid state junction materials have been developed and these are commercially available as thermoelectric modules from several vendors.
Thermoelectric modules are conventionally used to provide cooling of a heat transfer fluid, which in turn is used to provide heat transfer in cooling systems such as small refrigerators, air conditioners, cold traps for vacuum systems, cooling controls for thermocouple reference junctions, cooling devices for scientific equipment such as infrared detectors, cold stages on microscopes or on microtomes used for sectioning cooled tissues, and cooling electronic equipment. Thermoelectric modules can also be operated in reverse to convert heat energy into electric energy and have been used in power generation systems for spacecraft.
Thermoelectric cooling modules are commercially available in a variety of sizes and ratings. Cooling capacities range from 1 to 100 watts per module. A single stage module can typically generate 30-80xc2x0 F. temperature difference, depending on the heat load conditions. Custom cooling applications can require multiple modules or a variety of heat transfer surfaces. When multiple modules are used, the cooled surface of one module is placed in contact with the heated surface of the adjacent module.
The attractiveness of thermoelectric cooling devices is that they are rugged and reliable solid state devices with no moving parts. They are silent, have minimal maintenance requirements and have long lifetimes (around 200,000 hours). For small cooling loads, thermoelectric devices can be much lighter and more compact than conventional vapor compression chillers. The device can be made small and very rugged for portable applications.
Although the prior art includes various thermal precipitators as fine particle collectors and thermoelectric modules as heat transfer or power generating devices, there exists a need for a particle precipitation device capable of collecting fine and ultrafine particles that is miniaturized, portable and consumes less power than traditional precipitators. The present invention fills these and other needs, and overcomes the short-comings of the prior art.
It is an object of the invention to provide a thermal particle precipitator utilizing one or more thermoelectric modules that can be powered by batteries and do not require connection to water and external electrical sources at the collection site, so that the thermal precipitator can be readily transported and used at remote locations.
It is also an object of this invention to provide a thermoelectric particle precipitator using thermoelectric modules which can be of a small or miniaturized size so that the precipitator can be used as a nonobtrusive personal sampler.
It is another object of this invention to provide a thermoelectric particle precipitator as described which is of a small or miniaturized size and yet is of durable and rugged construction so that it can provide reliable operation even under extreme handling or environmental conditions.
It is a further object of this invention to provide a thermoelectric particle precipitator with a collection surface that integrates detection technology directly onto the collection surface so that in situ analysis can be performed on the collected particles.
It is a yet further object of this invention to provide a thermoelectric particle precipitator using thermoelectric modules that can be easily integrated into other equipment and used to produce clean air streams devoid of particles.
To accomplish these and other related objects, in one aspect, the invention is directed to a thermoelectric particle precipitator that removes and collects particles from a fluid stream using one or more thermoelectric modules. The thermoelectric module has first and second surfaces and is operable when the module is energized by direct current to cause cooling of the first surface and heating of the second surface. When a thermal mass is placed in a facing relationship to either the first or second surface of the thermoelectric module by a preselected and/or adjustable distance of separation, a temperature differential is formed between the thermal mass and the facing surface of the thermoelectric module. When the thermal mass is a heat source, it faces the cooled first surface of the thermoelectric module. Conversely, when the thermal mass is a heat sink, it faces the heated second surface of the module. A fluid flow passage is formed in the space between the thermal mass and the facing of the first or second surface of the thermoelectric module. An inlet is provided through which a fluid stream containing suspended particles is introduced into the fluid flow passage and an outlet allows the fluid stream to be removed from the fluid flow passage. The preselected distance of separation between the thermal mass and the facing surface of the thermoelectric module is effective when the temperature differential is formed to permit the particles in the fluid stream to undergo thermophoretic movement and collect on the cooler of the facing surfaces of the thermal mass and the thermoelectric module and thereby be removed from suspension in the fluid stream.
The thermal mass can be another thermoelectric module or, alternatively, it can be any other suitable source of heating or cooling, including ambient air. Movement of the fluid stream through the fluid flow passage can be induced by various means, including rotative movement of the one or more thermoelectric modules, fluid pumps and natural convection. The fluid stream can be ambient air or another gaseous medium, but may also include liquid mediums.
In another aspect, the invention is directed to a method of separating particles from suspension in a fluid stream using the described thermoelectric particle precipitator and then, optionally, analyzing the collected particles to determine their composition.